[css-shadow-1] Move definitions for `part` and `exportparts` attributes to host language specs

As mentioned by @Crissov in #13360, the [`part`](https://drafts.csswg.org/css-shadow/#part-attr) and [`exportparts`](https://drafts.csswg.org/css-shadow/#exportparts-attr) attributes actually define features for the host language.

Therefore, their definition should be moved to the HTML spec, the SVG spec and/or the DOM spec and only be referred to in the CSS spec.

@tabatkins @fergald

Sebastian